The present invention relates generally to apparatus and methods for sealing voids in a subterranean formation, and more particularly, the present invention relates to downhole tools that employ an outer tubing disposed around an inner tubing for placement of a sealant mixture into a void in a subterranean formation.
Sealant mixtures are commonly used in subterranean operations. Sealant mixtures may be used to seal voids in subterranean formations for a variety of reasons, such as to provide zonal isolation or to seal a lost circulation zone. For example, a sealant mixture may be used to form a seal in a void in a subterranean formation that prevents the undesirable migration of fluids between zones. Furthermore, sealant mixtures may be used for sealing abandoned underground voids, such as mineshafts, depleted wells, and the like. Sealant mixtures may also be used to seal a void, such as a mineshaft or mine entry, to suffocate and/or aid in putting out a coal fire.
One example of a sealant mixture commonly used in subterranean operations is a flowable cement composition. Flowable cement compositions generally comprise an aqueous-based fluid and hydraulic cement. Blends of hydraulic cement with fly ash, such as “POZMIX®” cement, may also be used. POZMIX® cement is an ASTM Class F fly ash cement that is commercially available from Halliburton Energy Services, Duncan, Okla. Generally, these flowable cement compositions are delivered to a void in the subterranean formation and allowed to set, thereby forming the desired seal. The use of these flowable cement compositions, however, may be problematic. For example, because these cement compositions are flowable considerable amounts of them may be wasted by flow into vugular porosity, natural fractures, weak formations, and other undesirable areas besides the desired void to be sealed. To account for the amounts of the flowable cement compositions wasted by flow into these undesirable areas, an excess volume of the flowable cement composition may be used. But this may add considerable expense due to the excess material needed and add additional uncertainty due to inaccuracies in determining the amount of excess material needed to account for the undesirable flow off.
To counteract these problems associated with flowable cement compositions, substantially non-flowable cement compositions may be used. Substantially non-flowable cement compositions generally comprise an aqueous-based fluid, hydraulic cement, and an activator (e.g., sodium silicate). Blends of hydraulic cement with fly ash may also be used. By using substantially non-flowable cement compositions, the waste and uncertainties associated with flowable cement compositions may be reduced, inter alia, because the substantially non-flowable cement composition does not flow away from the area of placement. Instead, the non-flowable cement composition begins to harden after placement without flow to undesirable areas.
A number of techniques have been developed for mixing and delivering the substantially non-flowable cement compositions into the desired location within the subterranean formation. In one such method, the components of the substantially non-flowable cement composition are first mixed on the surface. Next, the substantially non-flowable cement composition may be placed into the subterranean formation by pumping it through a delivery means (e.g., a conduit, a tube, or a pipe) to the opening of the void to be sealed for free-fall placement therein. However, pumping the substantially non-flowable cement composition into the subterranean formation may be problematic, inter alia, due to the pumping requirements associated with pumping this composition through the delivery means.
Another technique for mixing and delivering the substantially non-flowable cement composition to the desired location in the subterranean formation involves utilization of a two component system, whereby the two components of the substantially non-flowable cement composition are mixed downhole to form the desired composition. The first component generally may comprise an activator, and the second component generally may comprise a flowable cement composition, such as those described above. Alternatively, the second component may comprise the activator, and the first component may comprise the flowable cement composition. Shown in FIG. 1 is one such prior art technique for delivery of the substantially non-flowable cement composition to the desired location, e.g., void 100 in subterranean formation 102. This technique involves placing downhole tool 104 comprising tubing 106 into borehole 108 penetrating subterranean formation 102. Tubing 106 may be bull plugged (not shown) with a plurality of ports 110 disposed in the bull plug. In addition, borehole 108 may be lined with casing 112, which extends from the ground surface (not shown) into borehole 108 and terminates above void 100. Casing 112 may be cemented to subterranean formation 102 by cement sheath 114. Annulus 116 is formed between casing 112 and tubing 106. Furthermore, casing 112 should extend beyond tubing 106, forming mixing chamber 118 between the bottom end of tubing 106 and the bottom end of casing 112. In operation, the two components of the substantially non-flowable cement composition are delivered downhole simultaneously. First component 120 may be delivered down through tubing 106, out through ports 110, and into mixing chamber 118. Second component 122 may be delivered down through annulus 116 into mixing chamber 118. In mixing chamber 118, the two components combine to form the substantially non-flowable cement composition. After mixing, the substantially non-flowable cement composition is delivered to void 100 by free-fall drop from mixing chamber 118. Once delivered, the substantially non-flowable cement composition hardens to form a seal.
However, this technique has drawbacks. For instance, large volumes of the substantially non-flowable cement composition may be required because of imprecision in placing such composition in the desired location within the subterranean formation. Moreover, the borehole may no longer be in a usable state after formation of the seal due to plugging of the borehole by the seal. Additional problems may be encountered where the borehole is not centrally aligned over the center of the desired location, such as a mineshaft. This may result, inter alia, in premature sealing of the borehole prior to the sealing of the mineshaft.